1. Field of the Invention
This invention relates generally to the field of photolithography as employed for the fabrication of micro- and nano-structures, and it relates particularly to the field of photolithography based on the Talbot effect.
2. Description of Related Art
Lithographic fabrication enables the formation of micro- and nano-patterns on surfaces. Photolithographic techniques achieve this by exposing a photosensitive surface to a light-field with an intensity distribution corresponding to the desired pattern. The photosensitive surface is usually a thin layer of a sensitive material, such as photoresist, which is coated either directly on a substrate surface or indirectly over intermediate layers of other materials. Chemical or physical changes that occur in the photosensitive layer as a result of the exposure are used in subsequent processes to obtain a desired pattern in the material of the substrate or in an intermediate layer of another material. In the most commonly used photolithographic technique an image of a pattern defined in a mask is projected onto the substrate surface using an optical system.
For many applications patterns are required that comprise a unit cell of pattern features that repeat in one or two dimensions, that is, periodic patterns. A specialized photolithographic technique for transferring such patterns from masks onto substrates is based on the Talbot effect. When a periodic pattern defined in a mask is illuminated with a collimated beam of monochromatic light, diffraction orders in the transmitted light-field reconstruct “self-images” of the pattern at regular distances from the mask in so-called Talbot planes. For linear gratings the separation of the self-images, S, which is known as the Talbot distance, is related to the illumination wavelength, λ, and period of the pattern, p, byS≈2p2/λ  equ. (1)
Whereas, this formula has good accuracy when p>>λ (i.e. when the light is diffracted at relatively small angles), it approximates less well as the magnitude of p approaches A. Locating a photoresist-coated substrate at one of these planes results in the mask pattern being printed into the photoresist (see, for example, C. Zanke, et al., “Large area patterning for photonic crystals via coherent diffraction lithography”, J. Vac. Sci. Technol. B 22, 3352 (2004)). Furthermore, at intermediate distances between the self-image planes, Talbot sub-images are formed that have higher spatial frequencies than the pattern in the mask, which may be printed by placing a photoresist-coated substrate at one of these fractional Talbot planes. The printed results achieved using these techniques are improved when the duty cycle of the mask pattern (i.e. the dimension of the features as a fraction of the feature period) is selected to produce a high contrast of intensity distribution in the Talbot or fractional Talbot plane (see U.S. Pat. No. 4,360,586). It is also known in the prior art that the contrast of the Talbot images can be further enhanced by fabricating the periodic patterns in the mask using phase shifting materials. Photolithography using Talbot imaging is especially advantageous for printing high-resolution periodic patterns in view of the high cost of conventional, projection-type photolithographic systems for such patterns.
A major shortcoming of the Talbot technique, however, is that the intensity distributions of the self-images and sub-images are very sensitive to the distance from the mask, that is, they have a very narrow depth of field. This means that the substrate needs to be positioned very accurately with respect to the mask in order to correctly print the grating. This becomes increasingly more difficult as the grating period is reduced because the depths of field of the self-images and sub-images depend on the square of the pattern period. Furthermore, if the pattern needs to be printed onto a substrate surface that is not very flat or if there are topographical structures on its surface, or the pattern needs to be printed into a thick layer of photoresist, it may be impossible to achieve the desired result.
Achromatic Talbot lithography has recently been introduced as an improved method for printing high-resolution periodic patterns in a cost effective way (see H. H. Solak, et al., “Achromatic Spatial Frequency Multiplication: A Method for Production of Nanometer-Scale Periodic Structures”, J. Vac. Sci. Technol., 23, pp. 2705-2710 (2005), and U.S. Pat. Appl. no. 2008/0186579). It offers two significant advantages for lithographic applications: firstly, it overcomes the depth-of-field problem encountered using the classical Talbot method; and, secondly, for many pattern types it performs a spatial-frequency multiplication, that is, it increases the resolution of the printed features with respect to that of the pattern in the mask. In achromatic Talbot lithography (ATL) the mask is illuminated with a collimated beam from a light source with a broad spectral bandwidth, and beyond a certain distance from the mask the transmitted light-field forms a so-called stationary image whose intensity distribution is invariant to further increase in distance. In the case of a linear grating, the minimum distance, dmin, from the mask at which this occurs is related to the period of the pattern, p, in the mask and to the spectral bandwidth of the illumination, Δλ, by:dmin≈2p2/Δλ  equ. (2)
Beyond this distance, the Talbot image planes for the different wavelengths are distributed in a continuous manner with increasing distance from the mask, which generates the stationary image. Thus, by placing a photoresist-coated substrate in this region exposes the substrate to the entire range of transverse intensity distributions formed between successive Talbot planes for a particular wavelength. The pattern printed onto the substrate is therefore an average, or integration, of this range of transversal intensity distributions, which is substantially insensitive to longitudinal displacement of the substrate with respect to the mask. The technique therefore enables a much larger depth of field than with standard Talbot imaging, and a much larger depth of field than with conventional projection, proximity or contact printing.
The intensity distribution in an ATL image from a particular mask pattern may be determined using modelling software that simulates the propagation of electromagnetic waves through and after the mask. Such simulation tools may be used to optimize the design of the pattern in the mask for obtaining a particular printed pattern at the substrate surface.
The ATL method has been developed primarily to print periodic patterns that comprise a unit cell that repeats with a constant period in at least one direction. The technique may, however, also be successfully applied to patterns whose period spatially varies in a sufficiently “slow”, gradual way across the mask such that the diffraction orders that form a particular part of the stationary image are generated by a part of the mask in which the period is substantially constant. Such patterns may be described as being quasi-periodic.
A drawback of ATL is that it requires a light source with a significant spectral bandwidth in order that the separation required between the mask and substrate is not disadvantageously large. The angular divergence of the different diffracted orders propagating from the mask produces spatial offsets between the different orders at the substrate surface resulting in imperfect image reconstruction at the pattern edges, which becomes worse with increasing separation. Fresnel diffraction at the edges of the diffracted orders also degrades the edges of the printed pattern, and this likewise gets worse with increasing separation. For these reasons laser sources, which have relatively small spectral bandwidth, are in most cases unsuitable for ATL.
A difficulty with applying non-laser sources such as arc lamps or light emitting diodes to ATL is obtaining the combination of high power in the exposure beam for ensuring high throughput in a production process, and good beam collimation for ensuring high-contrast imaging and minimizing loss of feature resolution. Obtaining good collimation from non-laser sources requires spatial filtering of the output beam which generally results in a large loss of power.
The advantages of the ATL technique may be obtained using a different but related technique that is disclosed in U.S. Pat. Appl. no. 2008/0186579. In this scheme, the periodic pattern in the mask is illuminated by a collimated beam of monochromatic light and during exposure the distance of the substrate from the mask is varied over a range corresponding to an integer multiple of the separation between successive Talbot image planes in order that an average of the intensity distributions between Talbot planes is printed on the substrate. The smallest displacement that may be employed is therefore equal to the separation of successive Talbot planes (when integer=1). With this displacement during exposure, the pattern printed on the substrate is substantially the same as that printed using the ATL technique. It is disclosed that the displacement may be performed either continuously or in a discrete way by exposing the substrate at multiple discrete positions over the range. Using the continuous displacement, the speed of displacement is necessarily constant in order that the desired average of the transversal intensity distributions is obtained, and using the discrete, or stepped, displacement, the exposure dose at each discrete position should necessarily be the same for the same reason. The general technique may be referred to as displacement Talbot lithography (DTL)
Whereas the integrated intensity distributions generated at the substrate using the ATL and DTL techniques are essentially equivalent, and both enable a large depth of field and spatial-frequency multiplication for the printed pattern, the DTL scheme has the advantage that it can be used with much smaller separations of the substrate and mask. This reduces the degradation of the pattern edges and allows more efficient utilization of the output from the light source because of the less stringent requirement on collimation. Further, the DTL technique enables the use of laser sources, which may be preferred for production processes. The light from such sources can be formed into well-collimated beams with negligible loss of power, so minimize loss of feature resolution and maximize image contrast.
The structure of the patterns printed using DTL from a particular mask pattern may also be theoretically determined using simulation software.
The prior art further mentions that DTL, like ATL, may be applied to quasi-periodic patterns, though the details, limitations and disadvantages of this are not disclosed.
A drawback of the DTL technique is that the longitudinal displacement of the substrate relative to the mask during exposure has to correspond accurately to an integer multiple of the Talbot distance. When the displacement is exactly an integer multiple, the integrated intensity distribution exposing the substrate is independent of the initial separation of the substrate and mask, and so produces a uniform exposure of the pattern features on the substrate even if the mask and substrate are not accurately flat and parallel. If, on the other hand, the displacement is not an exact integer multiple of the Talbot distance because of, for example, mechanical hysteresis or limited stepping resolution of a displacement actuator, or because of inexact synchronization between the duration of the exposure by the illumination system and the displacement of the substrate, then the integrated intensity distribution depends on the initial separation. In this case, if the mask and substrate are not accurately flat and parallel, then a spatial variation of feature size is introduced into the printed pattern; or, if the mask and substrate are accurately flat and parallel but their separation is different for different substrates, then the size of the printed features varies from substrate to substrate; both of which may be problems for certain applications. These sensitivities of the printed feature size to the separation of the mask and substrate may be reduced by longitudinally displacing the substrate by a large number of Talbot distances relative to the mask, but this can introduce other problems such as degradation of the feature resolution (if the illumination beam is not well collimated), distortion of the feature shape (if the direction of displacement is not accurately longitudinal), degradation of the pattern edges (if the gap becomes too large), and disadvantageously requires a larger travel range in the mechanical system.
A further difficulty in arranging that the longitudinal displacement corresponds accurately to an integer multiple of the Talbot distance is that in the general case the transmitted light-field is not exactly periodic in the direction orthogonal to the mask, as is explained for two particular examples of one-dimensional and two-dimensional patterns below. In the case of a one-dimensional periodic pattern, i.e. a linear grating, if the grating period in relation to the illumination wavelength is such that only 0th and 1st diffraction orders propagate in the transmitted light-field, then the resultant interference pattern is exactly periodic in the direction orthogonal to the mask (neglecting effects at the edges of the mask pattern), and the self-image planes are well defined and separated by an exact Talbot distance. If, however, the period of the grating in relation to the wavelength is such that 2nd and possibly higher diffraction orders also propagate, then the phases of the higher orders at the self-image planes (as defined by the 0th and 1st orders) are not exactly the same as in the plane of the mask, and so self-images are not accurately formed and the transmitted light-field is not exactly periodic in the direction orthogonal to the mask. With higher diffraction orders it is therefore not possible with the prior-art teaching of DTL to avoid some dependence of the integrated intensity distribution on the initial value of the separation between the substrate and mask, which makes it difficult to print a pattern uniformly and reproducibly. In the case of two-dimensional periodic patterns, there are further difficulties in obtaining an exactly periodic light-field in the direction orthogonal to the mask. For example, if the periods of the pattern components in orthogonal directions are different, then the Talbot distances relating to the respective components are also different, and so in the general case the transmitted light-field cannot be periodic with either Talbot distance in the direction of propagation. In a further example, if the pattern features are arranged on a square grid (so that the periods of the pattern components in the two directions are the same) and the pattern period is selected so that only 1st diffraction orders propagate (including (±1, ±1) diagonally diffracted orders) in the transmitted light-field, then the different Talbot distance associated with the diagonally diffracted orders also degrades the periodicity of the light-field in the direction orthogonal to the mask.
Yet another difficulty with the prior art teaching of displacement Talbot lithography is its application to quasi-periodic patterns whose period is not uniform but varies slowly over the pattern area or to mask patterns containing a plurality of discrete grating periods. With such patterns, it is not possible to illuminate the complete pattern and displace the substrate relative to the mask by an exact integer multiple of the Talbot distance that simultaneously satisfies the different periods; and therefore, for reasons explained earlier, it is not possible to print such patterns uniformly.
It is therefore a first object of the present invention to provide a method and apparatus related to displacement Talbot lithography for printing a periodic pattern of features uniformly and reproducibly onto a substrate from a pattern in a mask without requiring the substrate to be displaced relative to the mask by a distance that corresponds accurately to an integer multiple of the Talbot distance.
It is a second object of the present invention to provide a method and apparatus related to displacement Talbot lithography for printing a periodic pattern of features uniformly and reproducibly onto a substrate from a pattern in a mask that does not require a relative displacement of the substrate with respect to the mask that is greater than the Talbot distance by a large factor in order not to degrade unacceptably any of the resolution of the printed features, the shapes of the printed features, and the definition of the pattern edges.
It is a third object of the present invention to provide a method and apparatus related to displacement Talbot lithography for printing a one-dimensional periodic pattern of features uniformly and reproducibly onto a substrate from a one-dimensional mask pattern whose period in relation to the wavelength of illumination is such that 2nd or higher diffraction orders are generated in the light-field transmitted by the mask.
It is a fourth object of the present invention to provide a method and apparatus related to displacement Talbot lithography for printing a two-dimensional periodic pattern of features uniformly and reproducibly onto a substrate from a two-dimensional mask pattern whose periods in the different directions are not the same or which generates diagonally diffracted orders.
It is a fifth object of the present invention to provide a method and apparatus related to displacement Talbot lithography for printing a periodic pattern of features uniformly and reproducibly onto a substrate from a mask pattern whose period varies either continuously or step-wise across the mask.
It is a sixth object of the present invention to provide a method and apparatus related to displacement Talbot lithography for printing a periodic pattern of features uniformly and reproducibly onto a substrate from a mask pattern that does not require an exact synchronization between the exposure by the illumination system and the displacement of the substrate or mask.